1. Field of the Invention
The present invention relates to an optical pickup device for reading information from a plurality of discs having different reading wavelengths, more particularly, to an optical pickup device comprising a semiconductor laser device whose a plurality of light emitters emitting laser beams with different wavelengths were integrated and a photodetector disposed correspondingly.
2. Description of the Related Art
Conventionally, a DVD/CD reproduction device is known which shares each optical pickup device for a CD disc reproduction device and a DVD disc reproduction device, and reads information through different wavelengths to each other.
As can be understood by comparing structures of CD and DVD discs, the thickness of the protective layer for a DVD disc is about one-half that of a CD disc, therefore, where an optical pickup using a monofocal lens reproduces both discs, when an optical beam which is optimum to reproduce a DVD disc is converged on the information recording surface, since the protective layer of a CD disc through which an optical beam is transmitted is larger than that of a DVD disc, a spherical aberration occurs in the optical beam, which does not allow the optimum optical beam to be converged on the information recording surface of a CD disc. Therefore, there is an optical pickup with a biofocal lens which is capable of focusing on different points on the same straight line and irradiating 2 laser beams to form a beam spot corresponding to the size of each information pit.
This kind of optical pickup device comprises a light emitter to emit the laser beam with the optimum wavelength (780 nm) to read information from a CD having a comparatively low recording density and a light emitter to emit the optimum laser beam with the optimum wavelength (650 nm) to read information from a DVD having a high recording density, and alternatively drives and controls either light emitter by discriminating the type of the disc. A reflecting light generated by irradiating an information reading light onto a disc is irradiated onto a photodetector. In this kind of DVD/CD reproducing system requiring 2 light emitters, where either one of the light emitters is irradiated from the surface which is perpendicular to the other one of light emitters, it is necessary to hold a space in which an optical system is disposed, which increases in size of the device.
To prevent this optical pickup device from increasing in size, it is proposed that 2 light emitters emitting laser beams having different wavelengths for a DVD and CD are integrated and by using a semiconductor device emitting a laser beam alternatively and a photodetector corresponding to 2 light emitters, this semiconductor laser device is disposed such that straight lines connecting to each light-emitting point of a plurality of light emitters come in contact with the track tangential line of the disc to be reproduced (for example, refer to Japanese Patent Laid-Open Publication No. 2001-209957 and No. 2001-184698).
In the configurations as shown in the these patent laid-open publications, a plurality of laser beams having different wavelengths can be guided through a single optical path, which allows a compact space. However, more specifically, since the positions of 2 light emitters are different, it is necessary to take those positions to the optical axis of the objective lens into consideration. When the center of the light source does not coincide with the optical axis, the image height is high and some aberration exists. Since this aberration may affect reading signals adversely, it is recommended to reduce it as much as possible. In addition, since the NA of a DVD is larger than that of a CD and on the contrary, the beam spot of a DVD is smaller than that of a CD, the aberration upon reproduction of a DVD is larger than that upon reproduction of a CD regardless of the high image and in addition, the rate of increase in aberration upon reproduction of a DVD is also larger in comparison with that of a CD. Accordingly, the semiconductor laser is configured such that a 650 nm light emitter is disposed on the center axis of the optical system or the distance from the 650 nm light emitter to the center axis of the optical system is smaller than that from a 780 nm light emitter to the center axis of the optical system.